Hot cams and smoothbores - how should it idle?

I picked up this KZ900 this past summer. The previous owner had it for about 15 years and it looked like a mess. I've been cleaning it up haven't opened up the motor though.

According to the previous owner it has an 1105cc big bore, hot cams and 29mm smoothbores. I can confirm it has the smoothbores.

Well I rode it once before buying it, the bike was much faster than my stock KZ and it didn't like to idle much. It would either idle high or barely idle at all. Now that I've gone through the carbs and rejetted them, then bench synced, the problem is still there. When it's warm it will idle around 2,500 rpm or if I move the idle screw down it will idle at 1,000 for awhile and then die if I don't blip the throttle.

I'm kind of wondering if this is what to expect with these cams? I don't have the specs but the guy said the head and cam cover had to be clearanced for them to fit, so I think they're lumpy. And I've also read that smoothbore idle circuits aren't as refined as 26 and 28mm Mikuni's.

Can I get opinions from other owners who have built motors and smoothbores?

Still some work to do, maybe one or more of these tasks will help:

Sync the carbs

Check the ignition timing

Check valve clearances

Thanks, Mike

sounds more like a vacumm leak. spray some wd40 around the intake boots while running.
if u hear a dif in rpm while spraying, u have a leak.
thet sould idle fine, some may like a slightly higher rpms, but the rpm should not change when it gets hot.

Hi Mark,

Thanks - I forgot to mention I checked each intake boot, put new rubber caps on the sync ports and silicone sealed the boots to the head.

Might as well try the WD40 just in case.

yes do try the wd40. youd be surprized at how it can detect a minor leak.
ggod luck.

Kapahulu wrote: ...silicone sealed the boots to the head....

Silicone likely won't last very long as a sealant in this location.

Regarding the smoothbores, if not already done, would remove and clean the air jet.



Good Fortune!

Okay I did not clean the air jets - I'll give that a try!

Be sure the screw driver tip is a perfect fit into the jet.

And if the air jet seems stuck in position, might also consider a lengthy soak with Kroil, PBBlaster, Liquid Wrench, or other penetrating oil.

Good Fortune!

I took the carbs off and one air jet came out easily. The jet was clear and I sprayed carb cleaner down the passage. The other three jets were stuck and I just blasted them with the carb cleaner, not wanting to soak them yet.

With the carbs back on, the bike still idles high or barely idles. I sprayed WD40 around the intake boots and no change in rpm's.

Next I might put another set of carbs on and see what happens, or soak the air jets on these smoothbores then remove and inspect them.

29 smoothbores should produce a strong steady idle at 1200 rpm.

Perhaps another cleaning (see images below), manometer sync, and fine tuning with pilot air screws would be in order.

When fine tuning the pilot circuit for best idle, should attain highest rpm via air screw adjustment, then use the thumb screw to lower the slides. Do this for each individual carb, and repeat until no pilot air screw adjustment will raise rpm.

Do not use the pilot air screws to reduce idle rpm. The correct pilot air screw adjustment is the screw position that produces the highest idle rpm.

When performing the bench sync, be sure the idle thumb screw has sufficient thread remaining available by which to later lower the slides during the later running sync and fine tuning.
Where this hasn't been done, the tuner may erroneously attempt to further reduce idle rpm by turning the pilot air screws (which simply screws up the pilot mixture).

And would also confirm valve clearances being within specs.

If not already done, would assure fuel mixture from the pilot circuit exits freely through the orifice into the bore.





Good Fortune!

Patton, your help is just fantastic. If I recall correctly, when I change the pilots and mains I did not go through this cleaning procedure. I'll take them off and give this a try.

I was about to cleanup an old set of 26mm's and install them to see if the problem would go away. This is a more thorough approach.

Aloha, Mike

Well Patton, I cleaned the pilot circuits per the pics above and saw a steady flow of cleaner coming out of #6 for each carb. Also re-sprayed cleaner into the air jet inlet.

Result - the same, doesn't idle well.

Next I guess I'll check the ignition timing and hookup the carb sync. And eventually I'll get around to checking the valve clearances.

BTW - when I had the carbs apart, I measured the float levels and they were 23mm from the gasket surface.

You had this right off the start for you may be chasing a gremlin until this is accomplished....
Sync the carbs
Check the ignition timing (timing light for 1/4 & 2/3 idle & @ 4Krpm)
Check valve clearances
Insure ignition components, coils, plug wires, sparkies etc... are within specifications

Just sayin...